1. Field of the Invention
The present invention is related to planar spiral slot antennas and, more particularly, to such antennas having a wide bandwidth.
2. Background Information
Antenna design requirements differ depending upon the particular application of the antenna. Recently, there is a demand for antennas which have the capability of acquiring RF signals from various satellite ranging systems. For example, the satellite ranging systems include the United States Global Positioning System (GPS), the Russian Federation GLONASS System, the European GALILEO System, and commercial services such as the OmniSTAR® System, which provides GPS enhancement data via satellite.
The various satellite ranging systems use signals in different frequency bands, which range from 1175 MHz to 1610 MHz. Thus, a wide bandwidth is required for an antenna designed to receive signals from different ranging systems, and in particular for an antenna designed for use with all of the systems.
There are some known wide bandwidth antennas, however, these antennas tend to have a three dimensional architecture comprised of a stack of individual planar antennas or a complex patch antenna structure. In either case, the three dimensional nature of the design leads to a high profile antenna which is not suitable for aircraft or other applications in which a small form factor is a critical feature or a desirable feature.
In addition to a low profile physical structure, it is highly desirable that a multi-mode ranging application (i.e., GPS, GLONASS, GALILEO, OmniSTAR®-L5,) antenna have a common phase center for the incoming signals at the various frequencies (e.g., from 1175 MHz to 1610 MHz). This is important because the positioning measurements from the various ranging systems are calculated with reference to the phase center of the antenna. Although there are known processes for correcting phase center variation when the geometric phase center of an antenna and the electrical phase center of that antenna are misaligned, any such misalignment must be minimal for high accuracy multimode ranging applications. For example, in many applications, geodetic measurements must be accurate to the millimeter level. However, typically, a common phase center has not been provided even with an error within an acceptable tolerance range by the wide band, three-dimensional antenna structures discussed previously.
A commonly owned U.S. Pat. No. 6,452,560 issued on Sep. 17, 2002, to Kunysz for a SLOT ARRAY ANTENNA WITH REDUCED EDGE DIFFRACTION, which is incorporated herein by reference, describes a low profile slot array antenna in which the geometric and electrical phase centers are aligned. A conductive layer on the front antenna surface includes the array of slotted openings. When an electromagnetic signal is fed into one end of a transmission line and sequentially coupled into the slotted openings, a corresponding signal is emitted from the antenna substantially in the direction of the antenna axis. The front antenna surface also includes a surface wave suppression region enclosing the slotted array and a plurality of through openings disposed between the surface wave suppression region and the peripheral edge of the antenna to reduce defraction of the emitted signal at the peripheral edge. This antenna is particularly useful in the United States Global Positioning System as its slotted openings are tuned to receive both the L1 and L2 frequency bands. However, the antenna was not designed to receive a wider bandwidth including satellite ranging signals from the other systems previously mentioned.
It is also important in antenna design to provide an improved gain at low elevation signals, while still maintaining multi-path rejection. Reduced signal variation is also important in the azimuth plane at low elevation angles for L-band signals in the 1520 to 1560 MHz range.
It is thus an object of the invention to provide an antenna which has a wide bandwidth and a common phase center across the frequency band of interest. Additionally, it is an object of the invention to provide reduced signal variation in the azimuth plane and low gain at low elevation angles, and improved polarization purity.
Other objects of the invention will be apparent from the following detailed description.